Hybrid electric vehicles (HEVs) and Plug-in HEVs (PHEVs) run primarily in an engine-off mode where the vehicle is propelled by an energy storage device (e.g., a battery) and/or an electric motor. The vehicle may be operated in an engine-on mode only during selected conditions. As a result, depending on the vehicle drive cycle and the battery charging cycle, the engine may be operated infrequently. While reduced engine operation times in hybrid vehicles enable fuel economy and reduced fuel emissions benefits, the shorter engine operation time also generates insufficient heat for keeping an exhaust catalyst activated. This, in turn, results in higher vehicle cold-start emissions.
One approach to address the cold-start emissions is illustrated by Hanada et al in U.S. Pat. No. 6,427,793. Therein, during vehicle operation, a hybrid vehicle is switched between different travel modes based on catalyst activation. Specifically, when the catalyst is activated, the vehicle is operated in a normal travel mode with engine operation based on torque demand. In comparison, when the catalyst is not activated, the engine is turned on irrespective of the operator torque demand to heat the catalyst.
The Inventors herein have recognized that the approach of Hanada et al may lead to frequent engine running which degrades the efficiency and core purpose of a hybrid vehicle. In particular, since engine operation with a cold catalyst degrades exhaust emissions, the engine is pulled-up frequently to keep the catalyst sufficiently hot, thus making it difficult to balance exhaust emissions and engine run-time in a hybrid vehicle. In addition, the vehicle operator's drive feel may be reduced due to the engine being pulled-up when the operator does not expect it to.
In view of these issues, the inventors herein have developed an approach to enable reduction in cold-start emissions as well as to reduce engine run time in a hybrid vehicle. In one embodiment, the method includes: within a single vehicle drive cycle, initially pulling up an engine to, and holding at, a higher power in response to operator torque demand, and subsequently pulling up the engine to, and holding at, a lower power in response to catalyst temperature. In this way, the engine-off duration can be extended.
In one example, a hybrid vehicle may be operated in an engine-off mode (or electric-only mode) until a driver torque demand exceeds the power that can be provided by a system battery and/or motor. To meet the increased power demand, the engine may be pulled-up to a higher power (e.g., a higher engine speed and a higher peak torque). The engine may then be maintained at the higher power for a duration, irrespective of the driver torque demand, in a first cold-start mode to opportunistically heat an exhaust catalyst during this initial engine cold-start. Once the catalyst is heated, the engine then be pulled down and the vehicle may resume operation in the engine-off mode.
While the vehicle is operating with the engine off, exhaust catalyst cooling may occur. For example, due to cold ambient conditions and/or the vehicle sitting still (e.g., in a parking lot, or in a tunnel), the exhaust temperature may fall to or near a (lower) temperature threshold, below which the catalyst's activity is degraded. In response to catalyst cooling, a subsequent engine cold-start operation may be performed. Specifically, the engine may be pulled-up again, however to a lower power (e.g., a lower engine speed and a lower peak torque) in a second cold-start mode. The engine may then be maintained in the lower power for a duration to sufficiently heat the exhaust catalyst. In addition, the threshold temperature at which the engine is pulled-up may be adjusted based on the battery's state of charge with the threshold lowered when the battery has more charge. This reduces the frequency with which the engine is pulled-up to heat the catalyst when the battery can meet the power demand while taking advantage of a more frequent engine pull-up when the battery is depleted to meet the power demand and synergistically heat the catalyst.
In this way, by initially operating an engine at a higher power to pre-heat the catalyst and subsequently operating the engine at a lower power to enable sufficient catalyst heating, an engine-off duration over a given vehicle drive cycle can be extended. By reducing the need for frequent engine pull-up, engine operation time is reduced, improving the efficiency and fuel economy of a hybrid vehicle. By intermittently heating the exhaust catalyst, the catalyst is maintained active, reducing cold-start emissions when the engine is operated. Overall, engine cold-start emissions can be balanced with reduced engine run time of a hybrid vehicle.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.